Carbon dioxide is known to promote plant growth, but its effects on other aspects of crops are poorly understood. In many cases, the benefits of increased CO2 in the atmosphere will be offset by heat stress, drought and extreme weather tied to climate change. Credit: Schellack via Wikimedia Commons

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A carbon dioxide-rich atmosphere could strip important food crops of their nutrients, a new study suggests.

Wheat, rice, barley and certain legumes like soybeans are classified as C3 plants, which corresponds to their ability as plants to convert carbon dioxide into energy. These C3 grasses and legumes have been shown to lose up to 15 percent of zinc and iron, the top two minerals in the human body, in experiments that artificially enhanced the concentration of carbon dioxide. These elements are crucial for a healthy immune system, cell development, hemoglobin production and brain function.

"Global iron and zinc deficiencies are an enormous public health problem," said Samuel Myers, lead author of the paper and a physician and research scientist at the Harvard School of Public Health. A lack of sufficient zinc and iron in the diet has led to the loss of about 63 million life years annually, Myers said.

The researchers placed C3 crops, as well as C4 crops like corn and sorghum, into open-top chambers in free-air CO2 enrichment (FACE) experiments in the United States, Australia and Japan. The FACE chambers were filled with CO2-laden air with concentrations of up to 584 parts per million. The current concentration of CO2 in the atmosphere is around 400 ppm.

Carbon dioxide is known to promote plant growth, but its effects on other aspects of crops are poorly understood. In many cases, the benefits of increased CO2 in the atmosphere will be offset by heat stress, drought and extreme weather tied to climate change. A recent paper in Environmental Research Lettersfound that hotter-than-normal weather during a corn crop's critical flowering period could wipe out any benefits tied to CO2 (ClimateWire, March 20).

Although C4 crops were less affected than C3 crops, the nutritional values of the latter were still affected. C3 crop yields tend to increase more with CO2 concentration, as they are less efficient in using carbon dioxide than C4 plants and benefit more from the boost. But the increase in production is not likely linked to a lower relative amount of nutrients, because the responses from different varieties were so different.

Some types of rice became slightly more nutritious, for example.

"It is not as simple as it being a dilution," said Andrew Leakey, a co-author of the paper and an associate professor of plant physiology at the University of Illinois, Urbana-Champaign.

Other unknown factors involved
This means there are other factors at play, Leakey said, although they remain a mystery and an area of promising future research. This paper represents the first large-scale data set for such an experiment to come out of the University of Illinois.

About 2.4 billion people derive at least 60 percent of their food from C3 crops, Myers said. The breakdown between C3 and C4 plants in terms of global calories consumed depends on one's geographic location. Americans and Africans eat more C4 crops like corn, sorghum and millet, while Asian diets consist primarily of rice, a C3 grass.

"Having identified the problem, this is something we should direct energy toward solving," Leakey said.

Efforts are underway to develop crops that deliver more iron and zinc, as well as other necessary nutrients, Myers said. But it's a complex process to fortify crops using plant breeding techniques, genetic engineering and better crop management practices.

Mourad Moursi, a fellow with the nonprofit research organization HarvestPlus, said biofortification of crops with nutrients has made strides in the past few years. HarvestPlus has successfully developed a sweet potato high in vitamin A, iron-rich beans, and wheat and rice with higher levels of zinc for consumers in Africa and Asia. In some cases, the level of nutrients can increase by 70 percent.

"We were not out to specifically counter the effects of climate change," said Moursi, explaining the development of these crops. "It was really to provide another tool in the box of tools for reducing malnutrition in the developing world."

HarvestPlus received $29.5 million from donors in 2012. Restrictions on genetic engineering can limit the advances, Moursi said.

"We would like to drive [nutrient levels] up as high as we can," Moursi said, but HarvestPlus uses conventional methods because of regulatory and trade restrictions on sending genetically modified crops to Asia and Africa.

ABOUT THE AUTHOR(S)

Tiffany Stecker

Tiffany Stecker writes about climate change, agriculture and forestry for ClimateWire in Washington, DC and prefers red potatoes to Yukon Gold. She tweets about science, environment and music @TiffanyStecker.

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